Because there are several advantages using electromagnetic radiation with high frequency several approaches are known in the state of the art to achieve radiation in THz-frequency.
Many of the frequencies and wavelengths of importance to the spectroscopy of molecules and solid bodies may be within the wavelength range extending from 3 mm to 30 μm from 100 gigahertz to 10 terahertz. The use of a micro-radiation source, which may be tunable within the wavelength range and implemented on a semiconductor chip of a wafer for this range of terahertz radiation and which exhibits sufficient power output within the range of between 1 μW and 1 W, may be substantially significant from a technical standpoint for spectroscopic applications in all areas of environmental protection, analytics, and in material characterization in military fields, medicine and biology, as well as in chemistry and physics. In addition such a miniaturized source is needed in building source arrays for security applications like body scanners and applications of array scanners using THz radiation.
The terahertz range of the electromagnetic spectrum lies between the infrared and radiofrequency regions of the spectrum. Recent technological advances have allowed for exploration of the possible applications within this region of the spectrum. The electromagnetic radiation in the terahertz region has many potential applications including medical imaging and security.
Terahertz spectroscopy can be used for detecting and identifying biological, chemical and explosive materials. The spectroscopic database in the terahertz range of the electromagnetic spectrum is currently being compiled by labs throughout the world. In order to ensure safe employment of terahertz sources for such applications, the spectroscopy of biological materials and the interaction of terahertz frequencies with biological materials need to be studied in greater depth. Data of terahertz interaction with skin currently exists only from 0.1 to 2 THz.
The remaining portion of the terahertz spectrum—2 to 30 THz—remains unexplored. To enable the fielding of terahertz sources in many groundbreaking applications, it is necessary to further study the terahertz portion of the spectrum. A terahertz source and spectrometer would enable the biological research necessary to better understand the interaction of terahertz frequencies with biological tissue. The requirements are for a tunable terahertz source capable of producing energy from 0.1 to 7 THz at power levels of 1 W, CW. It is of importance to develop and produce powerful, tunable, affordable, and in addition miniaturized sources.
To generate certain frequencies in the far infrared range, coherent radiation for example, molecular lasers which are pumped by CO.sub.2 lasers may be used.
Other sources generate a THz pulse power using very high electron energies e.g. several MeV—Millions electron volts—and wigglers, which are alternating solid state magnets which force the electron to wiggle around the axis and therefore emit electromagnetic radiation. Such sources are called free electron lasers and can be used in special high energy physics institutes by customers on an hourly basis for very high cost. In search for a small, possibly portable THz source such installations cannot be favored.
THz radiation sources of today use semiconductor diodes, so called GUNN diodes, which allow several GHz of oscillator frequency, disclosed in “H. Hartnagel et al. “Ballistic electron waves swing (BEWAS) to generate THz signal power”, FREQUENZ Vol 63. Vol 3/4 (2009) 60-62”. This radiation is then selected by special filters to extract higher harmonics of the base frequency and use this as “THz” i.e. 200 GHz radiation. The extracted power from such diodes is in the μ Watt regime. The usable primary diode current is limited by Joules heating. With the requirement for powerful sources in the upper GHz and lower THz regime, semiconductors cannot reach the goal. Currents in solid state and current densities are limited by Joules heating from the vibrating atomic lattice. This is a hard wall for such devices.
Another way to generate coherent radiation in the far infrared range with a higher power output is based on the so-called Smith Purcell effect. It was proposed in “S. J. Smith, E. M. Purcell, Phys. Rev. 92, 1069, (1953)”. This new principle is based on the Smith Purcell effect, and uses a free electron beam crossing a metal grating to influence on this grating a vibrating surface charge which emits coherently THz radiation. The interaction of the DC primary beam with the standing emitted THz wave leads to a bunching of the beam and to an enhanced emission of the dipole radiation. It provides for generating radiation similar to the method known from the “free electron laser”. Macroscopic electron sources and diffraction gratings having a 100 to 300 μm period may be used to generate a coherent radiation field of polarized radiation having up to 1 mW power. Such sources are also proposed in a miniaturized form in the U.S. Pat. No. 6,909,104 A.
The reference “Intensity of Smith-Purcell Radiation in the Relativistic Regime”, J. Walsh, K. Woods, S. Yeager, Department of Physics and Astronomy, Dartmouth College, Hanover, N.H. 03755, U.S., pages 277-279, discusses the theory of such Smith-Purcell radiation sources and, additionally, gives experimental results. The reference “A New Source of THz-FIR Radiation” in LEOS NEWSLETTER, February, 1999 by J. E. Walsh, J. H. Brownell, J. C. Swartz, Department of Physics and Astronomy, Dartmouth College, Hanover, N.H. 03755-3528 and M. F. Kimmitt, Department of Physics, Essex University, Colchester, UK, Jan. 7, 1999, pages 11-14, discusses the design and mode of operation of a radiation source in the terahertz region. The experiments showed the feasibility of the approach disclosed in “M. Goldstein, J. E. Walsh, M. F. Kimmit, J. Urata, C. L. Platt, Appl. Phys. Lett. 71, 452 (1997)”. It may be that these terahertz radiation sources are perfectly efficient, but they do not yet suffice for many analytical applications, and they are not yet miniaturized to a sufficient degree.
This effect is used and described in a miniaturized Smith Purcell THz-radiation source EP 1 186079 B1. This source is miniaturized by using the EBID technology and a self reproducing fabrication technique with 3-dimensional direct deposition of the key structures, as the electron emitter and the miniaturized focusing optics, like it is disclosed in DE 10302794 A1.
It is to note that THz sources based on the Smith-Purcell effect suffer from losses in the metal grating, and can theoretically achieve up to 1 mW output power.
It is an object of the invention to propose a device for radiating electromagnetic wave in the THz regime that comprises high output power.
The invention is based on the knowledge that Dynatron tubes are known to generate electromagnetic radiation.
Dynatron oscillators are known in the state of the art i.e. from a textbook which describes the mechanism of self excited oscillations and a principle how to obtain a current voltage characteristic which has a negative slope. This negative slope can be used to obtain a self excited oscillator and a radio waves emitting tube.
Furthermore it is known state of the art of FR 581 147 and DE 69 304 C that a dynatron tube may be used to directly produce electromagnetic radiation. The radiation power of those devices is not high enough to use it with THz frequency.
However, due to the technologies of the vacuum tube in times before the second World War only wavelength of the dipole radiation in the 10 MHz regime could be obtained, which then could be transmitted by additional transmitter radio tubes and wire antennas.
A Dynatron oscillator tube is therefore known since the 1930-ties. “H. Barkhausen, Dynatron in Elektronenröhren Band 1 (1945) S.75 and Bd.3 (1935), S.73ff, Hirzel Verlag Leipzig” discloses, that the frequency range which could be reached is limited to <10 MHz by the technology of tube fabrication. Electromagnetic radiation is emitted using broadcasting amplifier tubes and wire antennas. The broadcasted power of the electromagnetic radiation is limited by the resistive Joules losses which occurred by heating the antenna wires. A system efficiency of a Lecher 2 wire sender is reported to be 5%, see “H. Barkhausen, Elektronenröhren Band 3 (1935) S.109 u Hirzel Verlag Leipzig.
The Dynatron is a triode tube, which has an electron emitter at cathode potential, a strongly positive extractor grid and a less positive anode potential. The Dynatron uses the effect, that electrons which hit the anode release from there a certain number of secondary electrons. This ratio is dependent on the electron energy, the anode material, and with this also the emitted current. Rising the extraction potential it was observed, that an increasing secondary electron current, which flows from the anode to the grid reduces the tube current, and therefore the current voltage characteristic I/V-curve of the tube starts to fall. This gives instead of a positive slope of the I/V curve, a negative slope in the I/V curve. Using a resonance circuit in the anode circuit like an LC-combination—Inductivity and capacitor vibration circuit—enables in action with the negative part of the I/V curve to excite an oscillation. The Dynatron therefore was used as an oscillator in radio transmission stations. Due to radio-tube and fabrication technology, the oscillator frequency reached in the 1930 ties was in the upper MHz regime. With the technology turn to transistors and semiconductor circuitries a successful development of oscillators to the upper MHz and lower GHz regime was possible.
To overcome this difficulty Ken Shoulders made in 1961 the proposal to use free flying electrons in vacuum tubes “Shoulders, K. R. (1961). Microelectronics Using Electron-Beam-Activated Machining Techniques. In: Advances in Computers. Franz Alt, ed., Academic Press, New York, 135-293”.
However the victory of the transistors in all the fields of electronics stopped the development of vacuum electronic devices for information transfer. However vacuum electronics succeeded in developing Microwave power sources, Gyrotrons and other satellite transmission tubes of today.